1. Field of the Invention
The present invention relates to the preparation of N-cyanoethanimidic acid esters, particularly the methyl and ethyl esters thereof having the formula I: ##STR1## in which R is CH.sub.3 or C.sub.2 H.sub.5.
Such esters are of industrial importance as starting materials for the preparation of heterocyclic compounds, such as, for example, 2-amino-4-methoxy-6-methyl-s-triazine, from which important plant protection agents are prepared.
2. Description of the Prior Art
It is known to prepare N-cyanoethanimidic acid esters in a two-stage process from acetonitrile, gaseous hydrogen chloride, alcohol and cyanamide, as disclosed in German Patent Specification No. 3,411,203. In this process the corresponding ethanimidic acid ester-hydrochlorides are formed in a first reaction between acetonitrile, anhydrous hydrogen chloride and methanol or ethanol, and these ester-hydrochlorides are converted into the corresponding N-cyanoethanimidic acid esters in a second reaction with cyanamide, with the elimination of ammonium chloride.
It is a disadvantage of this process that requires the use of anhydrous, and extremely corrosive, hydrogen chloride gas. This not only necessitates the use of very expensive, corrosion-resistant special equipment, but also the use of special safety measures to protect the environment, which makes the process very expensive. Another disadvantage results from the fact that large amounts of ammonium chloride are produced in this process, which requires expensive disposal procedures.
A possible alternative, described in the literature, is the preparation of ethyl N-cyanoethanimidates by reacting triethyl orthoacetate with cyanamide in the presence of two molar equivalents of acetic anhydride (cf. in this connection K. R. Huffmann and F. C. Schaefer, J. Org. Chem. 28 (1963) page 1816). In re-working this process, however, it has been found that it is purely a laboratory method which cannot be applied on an industrial scale because of serious disadvantages. This is because by-products are formed in this reaction, comprising two moles of ethylacetate and two moles of acetic acid per mole of triethyl orthoacetate. The separation of these by-products from the reaction product, during or after the reaction, causes considerable problems. On an industrial scale, a temperature of 160.degree. to 190.degree. C. is required for the continuous removal by distillation of the acetic acid formed, and at this temperature ethyl N-cyanoethanimidate decomposes partially or undergoes rearrangement into N-cyano-N-ethylacetamide by a Chapman rearrangement. In addition, reaction mixtures containing cyanamide should not be heated above 120.degree. C. for reasons of safety. A further disadvantage is the relatively low purity of product, complete separation from the acetic acid and unreacted acetic anhydride causing enormous difficulties in spite of an elaborate vacuum distillation at 25 to 30 mbar. Finally, this suggested process cannot be used for the preparation of methyl N-cyanoethanimidate.